Why is my thin glass dome taking so much light away from what is inside?
Notice in your reference image that there are multiple light sources clearly visible through the reflection on the gold egg. With any type of reflective/metallic object, having objects/lamps to reflect is important when trying to make the object not look flat.

I made a few changes to your scene to fix up the lighting:


I enabled smooth shading and auto-smooth normals on the glass dome, which also allowed me to reduce the number of subdivision surface iterations down to 2. That helped reduce render times a bit.
You should also consider moving the subsurf modifier on the glass dome before the
solidify modifier. When you apply the subsurf after the solidify
modifier, you end up flattening the solidify modifier by quite a bit.
I increased the size of the lamp from 0.1m to 0.44m. Glass refracts/and reflects light, so some of the light rays that would have struck the teapot without the glass would be refracted away from those points on the teapot as demonstrated by the shadow on the ground. That contributes to the perception of the gold being flat/dark. It also lets the light comes in from more angles, so more rays can strike the glass at points that make
them refract onto the teapot.